The coxsackievirus and adenovirus receptor protein (CAR) serves as the cell surface receptor for group B coxsackieviruses and most adenoviruses, but the physiological function and ligand for this protein remain to be described. An affinity column was constructed with the recombinant extracellular domain of the CAR (rECAR) to isolate potential ligands by affinity chromatography. Immunoglobulins G and M were consistently isolated from human sera passed through the column, suggesting that the CAR may be an immunoglobulin-binding protein. Further investigation revealed that the affinity-purified immunoglobulins bound to rECAR-coated immunoassay plates, and the peroxidase-labeled rECAR bound the immunoglobulins on ligand-overlay blots. The peroxidase-labeled rECAR was incorporated into immunoprecipitates formed between the affinity-purified immunoglobulins and rabbit antibodies against human immunoglobulins, but not into immunoprecipitates formed between mouse IgG and rabbit antibodies against mouse IgG. The CAR present in HeLa cell lysates also bound to the affinity-purified immunoglobulins on Immobilon membranes, showing that the association is not limited to the recombinant protein. These results demonstrate that the CAR binds IgG and IgM present in serum, and reveal a direct interaction between the coxsackievirus and adenovirus receptor and the immune system.
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